Power Mac Quicksilver G4 continually crashing

I have a Power Mac G4 quicksilver with OSX 10.5.8 that has always been quite unstable and flakey, (freezing up and crashing out of the blue) but in the last few months has become almost unusable. I'm no kind of expert but I always thought it might be a processor problem. I'm just wondering if anybody can give me more info on this issue and can anything be done to help.